The invention relates to a measuring arrangement with a soot filter for use in flowing, soot particle-bearing gases and its use, as well as a method for monitoring the operability of a soot filter arranged in an exhaust gas conduit, wherein at least one component stream of an exhaust gas stream flows through at least one molded element, which is open-pored at least in the flow direction, and wherein the temperature of the molded element is measured with at least one temperature probe.
Soot filters are chiefly used for filtering of soot particle-containing exhaust gases from internal combustion engines, preferably from diesel motors. The soot filter becomes contaminated with soot particles in the course of time, and must then either be exchanged or cleaned. The cleaning is here the more economical method and can take place continuously or at a certain point in time. The time for a cleaning can be selected at fixed time intervals, or as a function of the loading of the soot filter.
U.S. Pat. No. 4,404,795 describes a method and a device for reducing the particle output in the exhaust gas of a diesel motor with the aid of a filter. With a rising number of soot particles in the soot filter, the free filter cross section drops, and the exhaust gas pressure upstream of the filter rises. The measurable exhaust gas pressure upstream of the filter is used to establish the time when the filter should be cleaned. Once the maximum allowable soot particle concentration in the soot filter is reached, then it is determined by a temperature probe when the filter temperature reaches the temperature which guarantees burning off the soot particles without an additionally applied quantity of heat. When this temperature is reached, the filter at the exhaust gas inlet is additionally heated to the combustion temperature of the soot particles, and, while adding secondary air to promote combustion, the bum off is started, and the filter is cleaned. Determining the point in time for cleaning the soot filter by a pressure measurement is an indirect measuring procedure and relatively subject to disturbance. Thus, a measured pressure increase, which is not to be attributed to a loading of the soot filter with soot, undeniably leads to a misinterpretation of the condition of the soot filter and to the conducting of an unnecessary cleaning. A cleaning of the filter by combustion of the soot should, however, only be conducted as often as is absolutely necessary, since it can lead to a defect or breakdown of the soot filter by overheating. Such a failure cannot, in turn, be recognized by this system itself.
There arises the problem of creating a measuring arrangement and a method with which a monitoring of the operability of the soot filter is made possible in a reliable and direct manner.
This problem is solved for the measuring arrangement in that the soot filter is allocated at least one soot sensor, which has at least one molded element, which is open-pored at least in the flow direction, at least one electrical heating element, and at least one temperature probe. The soot sensor makes possible a direct determination of the amount of soot on the soot filter.
The molded element can, for example, be subjected to through flow by a complete gas stream, which has soot particles, or instead only be subjected to through flow by a portion of the gas stream. The molded element should not take up 100% of the soot from the gas, thus not replace a soot filter. It is sensible in any given case for only a fraction of the soot to be taken up by the molded element subjected to through flow and, so to speak, for a representative portion of soot particles to be removed from the exhaust gas.
With respect to the large number of configuration possibilities of the sensor geometry of the soot sensor, care should be taken that conductive compounds as, for example, catalytically active material or the soot itself, do not lead to signal disturbances or short circuits, which can endanger a trouble free operation of the heating element as well as of the temperature probe. Possibly the use of one or more electrically-insulating, soot-impermeable layers between heating element and molded element or between temperature probe and molded element can be necessary for this.
The soot sensor can here be arranged upstream of the soot filter, in an exhaust gas bypass to the soot filter, or downstream of the soot filter. An arrangement in the soot filter is theoretically also possible, but an unintentional ignition of soot particles on the soot filter by the soot sensor must be ruled out. It is advantageous, however, if a first soot sensor is arranged upstream and a second soot sensor is arranged downstream of the soot filter.
By a molded element which is open-pored at least in the flow direction is quite generally to be understood an element with an open porosity or penetrating openings or holes in the flow direction, which can be present as ordered or unordered. Here, it can be a matter of a simple perforated sheet, a tube, a packet of fibers or wool, a porous ceramic, a porous glass, a porous thin layer, or the like. But even a very rough surface can be used as a molded element, which is open-pored in the flow direction. It is advantageous if the molded element is similar to the filter unit of the soot filter. Well suited, for example, is a ceramic molded element, which is open-pored at least in the flow direction, with a honeycomb construction, as typically used for catalytic converters in motor vehicles, as well as a foam ceramic. It is furthermore advantageous if the molded element is at least partially covered with a catalytically active material, for example with platinum. The ignition temperature of the soot is thereby reduced, and the energy requirement for the electrical heating element is diminished. Such a coating with catalytically active material is, for example, also typical in motor vehicle catalytic converters.
The electrical heating element and the temperature probe can be arranged directly on or in the molded element. The electrical heating element, the temperature probe and the molded element can also, however, be arranged on a carrier. In this connection, it is only important that the electrical heating element and the temperature probe be connected thermally conducting with the molded element.
The use of the measuring arrangement according to the invention in the exhaust gas conduit of a motor vehicle to test the operability of a soot filter especially suggests itself, since the soot sensor can be constructed with small dimensions and with low weight. But a use in other combustion facilities, in which exhaust gases with soot particles are generated, is also appropriate.
The problem is solved for the method in that a portion of the soot particles remains adhered on the molded element, and in that the molded element is heated to the ignition temperature of the soot by an electric heating element in defined time intervals, and in that a development of heat occurring with the combustion of soot particles is used as a direct measure for the operability of the soot filter. The molded element can, for example, be flowed through by the entire exhaust gas or instead be flowed through only by a portion of the exhaust gas. The molded element should not, however, replace a soot filter. It is sensible that, in any given case, only a fraction of the soot is filtered from the exhaust gas by the flowed-through molded element and, so to speak, a representative sample of soot particles is withdrawn from the exhaust gas.
Here, the time intervals, in which the molded element is heated with the electric heating element, can be selected as fixed. But variable time intervals, which can be selected on the basis of an evaluation of operating data, can also be appropriate. For a soot filter in the exhaust gas conduit of a diesel motor, this could mean, for example, that the heating of the molded element is started after a predetermined number of cold starts or as a function of the diesel fuel consumed. By operating data are accordingly to be understood generally information which relates to the generation of the exhaust gas and which can be set in some relationship with a development of soot in the exhaust gas.
First, it is possible that, after reaching the ignition temperature of the soot on the molded element, the electric heating element be operated at a constant heat output, that the development of heat arising from the combustion of the soot particles be measured with the temperature probe, that the temperature rise be evaluated as a direct measure for the combusted amount of soot particles on the molded element, and that the operability of the soot filter be determined therefrom. An intelligent control unit is necessary for this, which can convert the temperature rise into an amount of soot by a predetermined calculation routine. The amount of soot, which bums on the molded element, is proportional to the amount of soot, which has flowed by since the last heating of the molded element.
Second, after reaching the ignition temperature of the soot on the molded element, the temperature of the molded element can be held substantially isothermal by withdrawing the heat output of the heating element, and the decrease of the heat output can be evaluated as a direct measure for the combusted amount of soot particles, and the operability of the soot filter can be determined therefrom. Here as well, an intelligent control unit is necessary.
If a molded element is arranged in the flow direction upstream of the soot filter, then after reaching the ignition temperature of the soot, when a development of heat on the molded element occurs, the loading of the soot filter with soot particles can be determined.
After evaluating the temperature increase or the change in heat output and conversion into an amount of soot combusted on the molded element upstream of the soot filter, the loading of the soot filter itself is inferred. For this purpose, a correlation formula must be stored in the intelligent control unit, which contains the relationship between deposits on the molded element and the loading of the soot filter. If an amount of soot on the molded element is computed, which lies above an allowable, predetermined threshold value for the loading of the soot filter, then a cleaning of the soot filter is started by the control unit. If, however, a cleaning function is not planned for a soot filter, then upon reaching or exceeding the threshold value a notification, for example with a warning light or an acoustic signal, is emitted that the soot filter must be changed.
If, however, an amount of soot on the molded element is computed, which lies below an allowable, predetermined threshold value for the loading of the soot filter, then no cleaning of the soot filter or emission of a notification is started by the control unit, but instead the value calculated for the amount of soot is stored. A subsequently started, second determination of the amount of soot on the molded element, repeated at a certain interval from this first determination of the amount of soot on the molded element, must now be processed in connection with the first determination or the value stored for this purpose. The amount of soot calculated from of the second determination must be added to the stored value by the control unit, since in this case only the sum of both values in the correlation formula will provide the correct value for the loading of the soot filter. If the threshold value has still not been exceeded after the second determination, then the sum of the two determinations must be stored and used further for subsequent calculations in accordance with the formula above.
If a molded element is arranged in the flow direction downstream of the soot filter, a leak in the soot filter can be detected after reaching the ignition temperature of the soot upon occurrence of a development of heat, and a warning signal can be emitted which indicates the defect in the soot filter, if the development of heat exceeds a predetermined threshold value. Such leaks occur, for example, after a failure of the soot filter owing to breakage. After evaluation of the temperature increase or the change in heat output and conversion into a combusted amount of soot on the molded element downstream of the soot filter, the computed value is compared with a stored threshold value, which usually results from legally specified threshold values. If an amount of soot on the molded element is calculated, which indicates an insufficient filter action of the soot filter because of exceeding an allowable, predetermined threshold value, then a warning signal is emitted that a defect in the soot filter exists and that this must be changed.
If a first molded element is arranged in the flow direction upstream of the soot filter and a second molded element is arranged in the *flow direction downstream of the soot filter, then, after reaching the ignition temperature of the soot upon occurrence of a development of heat on the first molded element, the loading of the soot filter with soot particles can be ascertained, and after reaching the ignition temperature upon occurrence of a development of heat on the second molded element, a leak of the soot filter can be determined and a warning signal can be emitted, which indicates the defect in the soot filter, if the heat development exceeds a predetermined threshold value.
If two or more soot sensors are used, then the evaluation of the respective sensor signals need not take place in the same way at each of the soot sensors, but instead different evaluation methods can be used.
It is also possible to arrange one molded element in an exhaust gas bypass to the soot filter, where after reaching the ignition temperature of the soot upon occurrence of a development of heat on the molded element, the loading of the soot filter with soot particles can be ascertained. The evaluation here takes place as with an arrangement of the molded element upstream of the soot filter.
The following seven figures should provide an exemplary, detailed explanation of the measuring arrangement of the invention and the process. It should be expressly added that not only a planar construction of the soot sensor is possible, as it is shown here. The arrangement of the molded element on a rod or a tube or the use of a massive, self-supporting molded element is also possible.